An induction furnace is known which comprises
A TUBULAR INDUCTION HEATER DISPOSED VERTICALLY AND MADE OF A MATERIAL WHICH IS OXIDIZABLE AT HIGH TEMPERATURE.
A toroidal chamber encloses said heater, the chamber being constituted by an assembly of superposed wall-forming members including: a horizontal lower plate; an outer tube and an inner tube which are substantially co-axial and rest vertically on said lower plate with the heater disposed co-axially between the tubes and an upper plate resting horizontally on the tubes. The upper and the lower plates each are provided with an opening to the bore of the inner tube to enable objects to be passed through the heater for heating.
An induction coil is disposed around the outer tube for receiving high-frequency electric current to induce heating eddy currents in the heater.
Further, means are provided for introducing an inert gas into said chamber at sufficient pressure for the gas contained in the chamber to escape through gaps between the contact surfaces of the said superposed wall-forming members.
In this furnace, the gaps between the superposed members must be of small cross-section to prevent air from entering the chamber during operation. To do this, the upper plate is evenly supported in practice not only on the outer tube but also the inner tube.
In general, the outer and inner tubes as well as the upper plate are made of the same refractory material such as silica, this making it possible to reach a maximum operation temperature of about 1600.degree.. When it is required to produce a furnace which operates at a higher temperature, the inner tube is made of zirconia or thoria for example. Because of their high price, these materials are used only for the inner tube, the silica outer tube being protected against radiation of the induction heater by interposition of a thermally insulative tube. But zirconia and thoria have relatively high coefficients of expansion and while the temperature of the furnace is rising, it appears that the upper plate laid on the inner tube rises because of the longitudinal expansion of this tube. A large gap then appears between the upper plate and the outer tube which causes air to enter and consequently rapidly damages the induction heater which is made of an oxidisable material such as graphite or tungsten.
To mitigate this disadvantage, it has been proposed to provide a disposition in which the upper part of the inner tube is engaged in a groove formed in the upper plate, an axial gap being provided at ambient temperature between the end of the inner tube and the bottom of the groove, this gap having sufficient size to compensate for the difference of expansion between the silica and the zirconia. But it then appears that a radial clearance must also be provided between the end of the wall and the groove to account for the radial expansion of the inner tube. In these conditions, the radial gaps and axial gaps which must be provided at ambient temperature between the inner tube and the groove in which it is lodged are too great and permit oxidation of the induction heater during the rise in temperature of the furnace.
The present invention aims to mitigate these disadvantages and to provide an induction furnace which can operate at a high temperature.
The present invention provides a high-frequency electric furnace comprising:
a tubular induction heater disposed vertically and made of a material which is oxidizable at high temperature;
toroidal chamber enclosing said heater, the chamber being constituted by an assembly of superposed wall-forming members including: a horizontal lower plate; an outer tube means and an inner tube means which are substantially coaxial and rest vertically on said lower plate with the heater disposed co-axially between the tube means and an upper plate resting horizontally on the tube means; the upper and the lower plates each being provided with an opening to the bore of the inner tube means to enable objects to be passed through the heater for heating;
induction coil disposed around the outer tube means for receiving high-frequency electric current to induce heating eddy currents in the heater;
and means for introducing an inert gas into said chamber at sufficient pressure for the gas contained in the chamber to escape through gaps between the contact surfaces of the said superposed wall-forming members.
The improvement resides in said inner tube means comprising an assembly of:
a first inner tube made of a first refractory material and resting on the lower plate;
a horizontally disposed washer resting on the upper end of the first inner tube; and
a second inner tube resting on the washer and whose upper end is engaged in a groove provided in the upper plate and sufficiently deep for a gap to remain between the upper end of the second tube and the base of the groove at ambient temperature; the second inner tube, the upper plate and the outer tube means being made of a second refractory material having both a maximum operation temperature and a coefficient of expansion which are lower than those of the first refractory material, differential expansion during heating between the first inner tube and the other wall-forming members being accommodated longitudinally by the depth of the said groove and radially by sliding on the washer, whereby the gas-passing gaps between the contact surfaces of the wall-forming members remain of substantially constant size throughout the temperature range from ambient to maximum operating temperature.